1. Field of the Invention
The present invention relates to a solid-state image pickup apparatus, its driving method, and a camera system, and particularly to a solid-state image pickup apparatus using an XY address type solid-state image pickup element typified by a MOS type image sensor, its driving method, and a camera system using an XY address type solid-state image pickup element as an image pickup device, such as a digital still camera.
2. Description of the Related Art
In an image pickup technique of a still picture typified by a digital still camera, a solid-state image pickup element including a large number of pixels is used as an image pickup device, and pixel information of all pixels is independently read out so that a still picture is obtained. For example, in the digital still camera, since a still picture taken by releasing the shutter is required to have high fineness, a solid-state image pickup element including more pixels is used.
On the other hand, before a still picture is taken, in general, an operation of confirming (monitoring) an object is carried out by projecting a motion picture (image of the object) on, for example, a liquid crystal monitor of a small screen. At the stage (monitoring mode) in which the object is confirmed, a rough image (image of low resolution) corresponding to the number of pixels of the liquid crystal monitor is satisfactory.
Besides, in the image transmission for a portable equipment such as a digital still camera, a data rate of transmission is limited. Accordingly, with respect to a still picture, pixel information of all pixels is transmitted to obtain an image of high fineness, and with respect to a motion picture, pixel information is thinned out to decrease the amount of information and is transmitted.
Hitherto, as an image pickup device for a single chip imager color camera, it has been general to use a charge transfer type solid-state image pickup element, for example, a CCD (Charge Coupled Device) type image sensor. On the other hand, in recent years, an XY address type solid-state image pickup element, which is more advantageous than the CCD type image sensor in view of reduction in consumed electric power and miniaturization of a system, for example, a MOS type image sensor, has come into use.
Incidentally, in the single chip imager color camera, in the thinning-out processing of pixel information in the case where the CCD type image sensor is used as the image pickup device, for example, a method has been adopted in which after pixel information for all pixels is read out from the image sensor, the pixel information is thinned out by an external signal processing system. Also in the color camera using, for example, the MOS type image sensor as the image pickup device, the thinning-out read method in the CCD type image sensor has been adopted.
The reason why the thinning-out read method has been adopted like this, in which the pixel information is thinned out by the external signal processing system after the pixel information for all pixels is read out from the image sensor, is as follows:
The reason is as follows: (1) it has been impossible to thin out the pixel information while the spatial arrangement of colors of color filters and the output sequence are held, (2) pixels are sequentially selected by a normal shift register used as selection means for selecting the pixels, (3) the CCD type image sensor has been able to read out signal charges from the pixels only in sequence, and so on.
As an example, when a case where pixel information is thinned out every other pixel is considered, if there is no color difference, the amount of information can be compressed by half in each of lengthwise (vertical) and lateral (horizontal) directions. However, as shown in FIG. 9, in the case where there is a color difference between respective pixels, if pixel information is thinned out every other pixel, pixel information is read out from only pixels of oblique line portions, and only pixel information of, for example, B (blue) can be read out, so that a color image can not be obtained.
From the reason like this, in the single chip imager color camera using the CCD type image sensor or the MOS type image sensor as the image pickup device, after the image information for all pixels is read out from the image sensor, the thinning-out processing of the pixel information has been carried out by the external signal processing system. However, in this case, in spite of the fact that the amount of information is reduced by the thinning-out processing, since a driving frequency of the image sensor is constant, consumed electric power can not be reduced, on the contrary, a load is resultantly applied to a subsequent stage signal processing system.
Especially, in the single chip imager color camera using the MOS type image sensor as the image pickup device, since its superiority over the CCD type image sensor is in the reduction in consumed electric power and the miniaturization of the system as described before, in the case where the amount of pixel information is reduced, if the consumed electric power is reduced without applying a load to the subsequent stage signal processing system in proportion to that, it can be said that the significance is very high.